1. Field of the Invention
The present invention relates to multiple axis vibration test platforms, and more particularly to a vibration test platform capable of providing single, dual, or three-axis fixed amplitude, variable frequency vibration.
2. Description of the Related Art
Vibration test equipment is widely used to test hardware to assure that it is developed to perform within the subjected environment. Vibration test equipment which can provide vibration in a single axis or in two axes are presently available. Vibration test equipment which can provide vibration in three axes is known but is not as readily available and requires complex control mechanisms to minimize cross coupling of motion in the various axes and cumbersome torque resisting apparatus to restrict rotation about the axes of vibration.
An example of a single axis vibration test platform is described in Wittkuhns U.S. Pat. No. 3,277,697 . The Wittkuhns vibration test platform provides for continuous change of both frequency and amplitude of vibration in a single axis. Such vibration is accomplished by use of a drive shaft and actuator arms coupled to the drive shaft. The actuator arms are coupled to the vibration test platform by means of eccentrics at the coupling to the drive shaft and at the coupling to the test platform. These eccentric couplings at both ends of the actuator arms of Wittkuhns will not eliminate cross coupling should the test platform be vibrated in more than one axis since the eccentric is fixedly connected to both the drive shaft and the test platform. Thus, the Wittkuhns design does not permit sufficient flexibility in the eccentric couplings at the ends of the actuator arms to permit vibration in more than one axis.
An example of a multiple axis vibration test platform is described in Jones U.S. Pat. No. 4,436,188 . The Jones vibration test platform utilizes a single hydraulic actuator along each of the respective axes of vibration with gimbal bearings disposed at each end of the actuator arms. Such gimbal bearings permit pure two-dimensional motion of each of the actuator arms at the respective couplings. Jones utilizes splines to restrict rotation of the test platform about an axis, however, such splines cannot restrict rotation about the third orthogonal axis since the gimbals only permit pure twodimensional motion. Thus, the multiple axis vibration test platform of Jones requires a very sophisticated control circuit to control each hydraulic actuator to sense the rotation of the table and counter such rotation by active forces along the respective actuators. Therefore, the multiple axis vibration test platform of Jones is extremely limited in its ability to provide vibration simultaneously in more than one axis since such vibrational motion would require counteractive forces to restrict rotation about a particular axis while also requiring a sophisticated control loop to control the amplitude of vibration of each hydraulic actuator.
A second example of a multiple axis vibration test platform is described in Cappel U.S. Pat. No. 4,265,123 . Cappel utilizes torquer tubes attached to each edge of the vibration test platform to restrict rotation of the test platform about each orthogonal axis. Cappel utilizes only single actuators along each respective axis to provide vibration of the test platform along that axis. Cappel suffers from the drawback that separate torque tubes must be provided in an effort to eliminate rotation about a respective axis.
A still further example of a multiple axis vibration test platform is provided in Thompson et al U.S. Pat. No. 4,446,742 . Thompson et al. utilize ball joints at each end of hydraulic actuators which provide for motion in each of the multiple axes. Thompson et al. further utilize torquer tubes on each axis to restrict rotation of the test platform about that respective axes. The Thompson et al. multiple axis vibration test platform suffers from the same drawbacks as do Cappel and Jones described above wherein the hydraulic actuators require sophisticated control loops to control the amplitude of vibration of each hydraulic actuator along each of the respective axes of vibration, and separate torque tube arrangements to prevent rotation about each of those axes while the platform is being vibrated.
With the foregoing drawbacks of prior art multiple axis vibration test platforms in mind, it is an object of the present invention to provide a multiple axis vibration test platform simple in construction and design which utilizes a relatively simple control loop to selectively vibrate the test platform in one, two, or three axes simultaneously.
It is a further object of the present invention to provide a simplified actuator arm arrangement for connection to a drive shaft and to the test platform wherein the actuator arm arrangement permits out of axis rotation of each arm while concurrently restricting vibration of the test platform about a respective axis.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.